Measurement of (18)O(18)O and (17)O(18)O in atmospheric O(2) using the 253 Ultra mass spectrometer and applications to stratospheric and tropospheric air samples

RATIONALE: The doubly substituted isotopologues (e.g., (18)O(18)O, (17)O(18)O) in atmospheric O(2) are potential tracers for ozone photochemistry and atmospheric temperatures. Their low abundances and isobaric interference are the major analytical challenges. The 253 Ultra high‐resolution stable isotope ratio mass spectrometer is suitable for resolving isobaric interferences. METHODS: O(2) from air is purified using gas chromatography on a packed column filled with molecular sieve 5 Å and cooled to −78°C. The δ(17)O, δ(18)O, Δ(17)O, Δ(35) and Δ(36) values are measured on the extracted O(2) with the 253 Ultra at medium mass resolution (M/ΔM ~10000) using Faraday detectors for the singly substituted isotopologues and ion counters for the doubly substituted isotopologues. RESULTS: Interferences from isobars, mainly (35)Cl for (17)O(18)O and H(35)Cl and (36)Ar for (18)O(18)O, are sufficiently resolved to enable high‐precision determination of Δ(35) and Δ(36). The Δ(35) and Δ(36) values of O(2) after photochemical isotope equilibration at −63°C and heating to 850°C agree with the theoretical prediction. The stratospheric Δ(35) and Δ(36) values are close to isotopic equilibrium at the ambient temperatures. However, the values for tropospheric O(2) differ from those expected at equilibrium. CONCLUSIONS: The 253 Ultra allows interference‐free clumped isotope measurements of O(2) at medium mass resolution. The Δ(35) and Δ(36) signatures in atmospheric O(2) are mainly governed by O(3) photochemistry, temperature and atmospheric transport. Tropospheric O(2) is isotopically well mixed and retains a significant stratospheric signature.